They Fit
We have an 84 H34. We added two batteries in the port settee. They were the 30 size battery. I left the two that charge from the engine or from a charger in the starbord lazarette. Added a second charger and perko switch for the additional batteries and ran them to the main DC connection in the panel. By selecting the perko's you can run the DC on the boat from any combination of the four batteries or all of them. Down side of this approach is that only two charge from the engine, but all charge when on shore power. Works well for us and gave us the extra power to run our refrigeration for a couple of days without recharging. If we keep the water fairly full the boat sits fairly level. Good Luck.